The present invention relates to methods and systems designed to control electric pumps. More specifically, the invention relates to a pump controller that measures a time difference between zero-cross inputs to control activation and deactivation of a pump.
Pumps for water and other liquids may be controlled in a variety of ways. The simplest and oldest method of controlling a pump is manual control, where an operator controls an on-off switch in response to the level of the liquid being pumped by the pump. Of course, manual control has several limitations, including the fact that in the absence of an operator the pump must either be left on, which often results in motor burnout or shortened motor life, or left off, which often results in liquid levels reaching unacceptable levels.
In response to these deficiencies, a number of automatic pumps and automatic pump controllers have been developed. Commonly, devices that sense the level of liquid, such as floats or sensors, are used to control a switch that when closed, connects the motor to a power supply, turning the motor on, and when open, disconnects the motor from the power supply, turning the motor off. While automatic devices such as these are an improvement over manual techniques, they are not completely satisfactory. Manual floats are often bulky and so are not useful in situations where pumps must be positioned in an area of limited space. Liquid level sensors must often be submersed or placed in close proximity to the liquid being pumped and are often corroded, fouled, or otherwise damaged by the environmental conditions. Accordingly, these types of pumps require a relatively high level of maintenance.
Because of these deficiencies, still other types of automatic pump control devices have been developed. Some pump protection devices exploit electrical phenomena that occur during the operation of an electric motor. When an electric-motor pump is initially turned on, the motor usually draws a relatively high-current signal. Thereafter, as the pump moves liquid, the motor current reaches a lower, relatively static level. If the pump encounters a xe2x80x9cno-loadxe2x80x9d situation, that is, a situation where the liquid level has dropped to such an extent that no more liquid is being drawn into the pump, the current drawn by the pump decreases and the voltage level stays at substantially the same level that it reached while under load. Many presently available devices that exploit these phenomena are deficient because they rely on switching and/or SCR-control methodologies, which generate harmonics that can overheat and, thereby, shorten the life of an electric motor.
Accordingly, there is a need for an improved apparatus and method of controlling pumps. The present invention provides a method and an apparatus for protecting an electric motor pump based on the phase difference of two zero-cross signals. In one embodiment, the method includes measuring the voltage of an AC power signal that has a positive half cycle and negative half cycle. The measured voltage is used to generate a first two-state output signal that 1) is in a logic high state when the measured voltage is on the positive half cycle and 2) is in a logic low state when the measured voltage is on the negative half cycle. The method also involves measuring the current of the AC power signal and generating a second two-state output signal that 1) is in a logic high state when the measured current is on the positive half cycle and 2) is in a logic low state when the measured current is on the negative half cycle. Once the two output signals are generated, the phase difference between the first and second two-state output signals is determined. The difference is then compared to a predetermined threshold value. If the determined phase difference is equal to or greater than the predetermined threshold value, a switch controlling the motor of the pump is opened. The switch is closed after a predetermined amount of time has elapsed.
The invention may be implemented in an apparatus that includes, in one embodiment, a voltage filter capable of being coupled to an AC power source and of generating a first two-state output signal, a current filter capable of being coupled to an AC power source and of generating a second two-state output signal, a power supply circuit capable of producing a DC power signal, a programmable device coupled to the voltage filter, the current filter, and the power supply circuit and operable to produce a control signal based on the phase difference between the first and second two-state output signals, and a switch coupled to the programmable device and operable to open in response to the control signal.
As is apparent from the above, it is an advantage of the present invention to provide an apparatus and method of controlling an electric motor pump. Other features and advantages of the present invention will become apparent by consideration of the detailed description and accompanying drawings.